28) Which of the following sets of ordered pairs does not represent a linear function?
(a) {(1, 2), (3, 4), (4, 5), (6, 7)}
(b) {(0, 3), (1, 5), (3, 9), (1, 13)}
(c) {(2, 2), (3, 3), (4, 4), (5,5)}
(d) {(1, 2), (2, 2), (3, 5), (4, 8)}